The present invention relates to a process for separation of effective components from polyester waste (which may be either valuable or valueless substances). More specifically, the present invention relates to a process for separation and recovery of effective components such as dimethyl terephthalate and ethylene glycol from polyester waste which contains polyethylene terephthalate as the major component along with a foreign material, with a high efficiency.
Because of their excellent chemical stability, polyalkylene terephthalates are produced and used in large quantities for common materials such as fibers, films, resins and the like, as well as in the field of food packaging, and as bottles for drinking water, carbonated beverages and the like.
However, the costs of processing fiber, film and resin product waste or off-specification materials such as polyalkylene terephthalates (hereinafter also referred to simply as xe2x80x9cpolyester wastexe2x80x9d), which are produced in large quantities with increasing production output and usage, are associated with higher product cost, while their processing is also a major problem for modern society, and therefore various recycling methods have been proposed for material recycling, thermal recycling, chemical recycling and the like.
Although material recycling whereby polyester waste is converted to low grade quality substances by melt molding has largely improved the situation for xe2x80x9cdisposablesxe2x80x9d, the obtained recycled products undergo further quality reduction when recycled again, and therefore their uses are limited and it has been difficult to avoid final disposal of polyalkylene terephthalates.
Another method employed is thermal recycling, whereby polyester waste is used as a fuel. This method offers the advantage of reutilizing the heat of combustion of the polyester waste, but since burn-off of the polyester waste cannot be avoided, it entails the problems of loss of the polyalkylene terephthalate starting material and generation of carbon dioxide, and is therefore not preferred from the standpoint of conservation of resources and preservation of the environment.
Chemical recycling is also being investigated as an alternative to the two types of recycling methods described above, whereby polyester waste is converted to its constituent components which are then recovered and again subjected to polymerization reaction to produce polyester for reuse.
Specifically, there is known a process in which the recovered polyester waste is reacted with methanol (hereinafter also abbreviated as xe2x80x9cMeOHxe2x80x9d) and is recovered as dimethyl terephthalate (hereinafter also abbreviated as xe2x80x9cDMTxe2x80x9d) and alkylene glycols, and a process in which the recovered polyester waste is hydrolyzed in the presence of an alkali compound and the resulting terephthalic acid and alkylene glycols are recovered. Such chemical recycling processes allow recycled reuse of compounds with essentially no loss, and thus offer advantages for reutilization of resources.
However, polyester waste delivered from the distribution industry and households usually contains foreign materials including chlorine-containing polymers such as polyvinyl chloride and polyvinylidene chloride (these will hereinafter be collectively referred to as xe2x80x9cPVCxe2x80x9d), colored polyester materials and polyolefins.
It has been difficult to avoid inclusion of such foreign materials even in the polyethylene/alkylene terephthalate portion (hereinafter also referred to as xe2x80x9cpolyalkylene terephthalate scrapxe2x80x9d) which is sorted from polyester waste. These included foreign materials may cause various problems such as generation of decomposition gases (for example, hydrogen chloride gas) during the heating and reaction operations for the polyalkylene terephthalate scrap, or may melt and solidify in the recovery apparatus leading to damage to the instruments or clogging and adhesion inside the instruments. In addition, formation of organic chlorine compounds from hydrogen chloride gas can notably lower the quality of the recovered DMT and alkylene glycols.
For chemical recycling, therefore, effective reuse of polyalkylene terephthalates contained in polyester waste has required separation of the foreign materials in the polyester waste.
A conventional process for recovery of polyester waste, which is widely known as the glycolysis/transesterification reaction process and which is employed industrially, is a process of depolymerizing the polyalkylene terephthalates with ethylene glycol (hereinafter abbreviated as xe2x80x9cEGxe2x80x9d) and then subjecting them to a transesterification reaction with MeOH to obtain DMT.
During removal of the foreign materials, however, thermal decomposition of PVC, for example, becomes notable from about 195xc2x0 C., and it has therefore been difficult to apply chemical recycling of PVC-containing polyalkylene terephthalate scrap as a result of the PVC decomposition that occurs by the above-mentioned glycolysis process that is usually carried out at 195 to 240xc2x0 C. Moreover, when chemical recycling is applied to polyester waste containing colored polyalkylene terephthalates, partial decomposition of the colored substances reduces the quality of the recovered compounds.
In addition, in the case of polyolefin-containing polyester waste, melting or solidification of the polyolefins occurs under the processing conditions for the conventional glycolysis/transesterification reaction, causing such problems as clogging of the apparatus and hampered operation and, therefore, difficulties have plagued the industrial treatment of polyolefin-including polyester waste by conventional polyester recovery processes.
As a means of solving this problem, U.S. Pat. No. 5,504,122 proposes a process of adding an alkali compound to hydrogen chloride formed by thermal decomposition of a chlorine-containing resin to sequester it, and then subjecting the polyalkylene terephthalate to methanolysis and recovering the starting monomer.
Also, Japanese Unexamined Patent Publication HEI No. 8-259728 proposes a process whereby a mixture of a chlorine-containing resin and polyester waste is hydrolyzed in the presence of an alkali compound and then terephthalic acid and EG are recovered, i.e., similarly, an alkali compound is added to sequester generated chlorine compounds.
While all of these processes achieve the object of recovering the target substances, they are associated with various drawbacks. Specifically, in cases where a chlorine-containing resin and its decomposition product are included as foreign materials, the chlorine compounds sequestered by the alkali compound must be removed, and the removal of the chlorine compounds requires the additional steps of distillation, washing and ion-exchange, thus complicating the process. In cases where a colored polyester is included as an foreign material, the high temperature treatment of the polyalkylene terephthalate decomposes part of the colorant and lowers the quality of the target substance. In cases where a polyolefin is included as an foreign material, melting treatment of the polyalkylene terephthalate together with its decomposition product makes it inevitable that the component to be recovered will be included in the polyolefin when it is removed from the thermal decomposition tank.
Japanese Unexamined Patent Publication HEI No. 11-21374 also proposes a process of hydrolyzing polyester waste in the presence of an alkali compound. However, since the decomposition is carried out in an alkaline aqueous system, this process has the drawback of a high reaction pressure.
Finally, Japanese Unexamined Patent Publication HEI No. 11-302208 proposes a process in which polyester waste is hydrolyzed in water-containing ethylene glycol in the presence of an alkali compound and the resulting sodium terephthalate is dissolved in a large amount of water and subjected to acid precipitation. According to this process, the presence of the ethylene glycol allows the reaction pressure to be lowered during the hydrolysis, but a drawback is encountered in that the acid precipitation produces a mineral acid salt in an amount almost equivalent to the terephthalic acid, and which is contained in the recovered terephthalic acid, or else the particle size of the produced terephthalic acid is reduced, leading to difficult handling of the powder during the polymerization process.
It is an object of the present invention to provide a process for recovery of dimethyl terephthalate and ethylene glycol from polyester waste (which may be either valuable or valueless substances) which contains polyethylene terephthalate as the major component along with foreign materials.
This object may be achieved by the following process according to the present invention.
The process of the present invention for separation and recovery of dimethyl terephthalate and ethylene glycol from polyester waste according to the invention is characterized in that a polyester waste containing polyethylene terephthalate as the major component along with foreign materials is successively subjected to the steps (a) to (f):
step (a) in which the polyester waste is mixed into ethylene glycol containing a polyester depolymerization catalyst, the resultant mixture is treated at a temperature of 175 to 190xc2x0 C. under a pressure of 0.1 to 0.5 MPa, and, from solid foreign materials contained in the resulting reaction solution, a fraction of the solid foreign materials which have floated to the surface of the solution, is removed by means of a floatation separation method;
step (b) in which, from the solution fraction delivered from step (a), the residual solid foreign materials which are contained in the solution and have not floated to the surface in step (a), is removed by a solid/liquid separation method;
step (c) in which the residual solution fraction delivered from step (b) is distilled and concentrated, to recover the distilled ethylene glycol;
step (d) in which the distillation residue delivered from step (c) is mixed with a transesterification reaction catalyst and methanol to cause a transesterification reaction between the distillation residue and methanol to occur and to produce dimethyl terephthalate and ethylene glycol, the reaction mixture is subjected to recrystallization treatment and then to centrifugal separation to separate the reaction mixture into a dimethyl terephthalate cake and a mixture solution, and the cake is subjected to a distillation purification, to recover the distilled dimethyl terephthalate, having a high degree of purity;
step (e) in which the mixture solution delivered from step (d) is subjected to a distillation treatment to recover the distilled methanol, and
step (f) in which the distillation residue delivered from step (e) is subjected to a distillation treatment to recover the distilled ethylene glycol.
In the separation and recovery process of the present invention, the depolymerization catalyst for step (a) preferably contains at least one metal compound selected from the group consisting of metal carbonates, metal carboxylates, metal oxides and metal alkoxides, and the amount of the catalyst is preferably controlled to 0.1 to 10% based on the weight of the polyester waste.
In the separation and recovery process of the present invention, the metal compound for the depolymerization catalyst preferably selected from the group consisting of sodium carbonate, sodium carboxylates, manganese acetate and zinc acetate.
In the separation and recovery process of the present invention, the amount of the ethylene glycol used in step (a) is preferably controlled to 0.5 to 20 times the weight of the polyester waste.
In the separation and recovery process of the present invention, the distilling and concentrating procedure in step (c) is preferably carried out under a pressure of from 1.33 kPa to 0.133 MPa.
In the separation and recovery process of the present invention, the foreign materials contained in the polyester waste may comprise at least one member selected from the group consisting of polyesters other than polyethylene terephthalate, or polyvinyl chloride, polyvinylidene chloride, polyolefins, polystyrene, acryl, rayon, acetate, polyvinyl alcohol, natural plant fibers, metals, pigments, oils, inorganic compounds, sand, paper, wood, glass, asbestos, carbon black and heat insulating materials.
In the separation and recovery process of the present invention, the polyesters other than polyethylene terephthalate which are contained as foreign materials in the polyester waste may comprise at least one member selected from the group consisting of copolymerized polyethylene terephthalates, polyethylene naphthalate, polytrimethylene terephthalate and polybutylene terephthalate.
In the separation and recovery process of the present invention, the polyolefins contained as foreign materials in the polyester waste may be polyethylene and/or polypropylene.
In the separation and recovery process of the present invention, the solid fraction which has floated to the solution surface in step (a) may contain a polyolefin and/or polystyrene.
In the separation and recovery process of the present invention, the solid foreign materials which are removed in step (b) may comprise at least one member selected from the group consisting of polyvinyl chloride, polyvinylidene chloride, unreacted polyester, acryl, rayon, acetate, polyvinyl alcohol, natural plant fibers, metals, inorganic compounds, sand, paper, wood, glass, asbestos and heat insulating materials.
In the separation and recovery process of the present invention, the ethylene glycol recovered in step (c) is preferably recirculated to step (a).
In the separation and recovery process of the present invention, the polyester waste may further comprise, as a foreign material other than polyethylene terephthalate, at least one member selected from the group consisting of polyamides, natural animal fibers, polycarbonates, polyurethanes, polylactic acid and dyes, and before step (a), a step (g) in which the polyester waste is mixed into ethylene glycol and treated at a temperature of 120 to 175xc2x0 C. under a pressure of 0.1 to 0.5 MPa to prepare a solution, the undissolved solid fraction is separated from the solution and the separated solid fraction is fed to step (a).
The method for reutilization of recovered dimethyl terephthalate of the present invention comprises reutilizing dimethyl terephthalate recovered by the separation and recovery process of the present invention as a starting material for production of terephthalic acid.
The method for reutilization of recovered dimethyl terephthalate of the present invention comprises reutilizing dimethyl terephthalate recovered by the separation and recovery process of the present invention as a starting material for production of bis(xcex2-hydroxyethyl) terephthalate.
The method for reutilization of recovered dimethyl terephthalate of the present invention comprises utilizing dimethyl terephthalate recovered by the separation and recovery process of the present invention, as a starting material for production of polyester.